Continuous tone images refer to images that have a virtually unlimited range of color or shades of gray. Photographs, for example, are continuous tone images. Digital hardware, however, is limited to expressing color or tone within a finite number of bits or even a single bit (i.e., “on” or “off”). Typically the continuous tone image is decomposed into an array of picture elements (pixels), each element capable of representing a finite number of colors or shades. Gray scaling and dithering are two processes used to approximate continuous tone images within the limitations of digital hardware. The digital representations of images simplify reproduction, storage, modification, and distribution of the images.
Once the continuous tone image is discretized into digital form, the digital form may be further processed to reduce storage requirements through compression. Compression eliminates information in a manner that results in either no visually perceptible differences or only acceptable visually perceptible differences between the digitized source image and the reconstructed image.
Some approaches to image compression entail direct manipulation of the spectral content. Reducing the spectral energy without regard to other factors frequently does not achieve optimal storage results due to rate influencing elements such as quantization and entropy encoding that are present in spectral content manipulating compression processes.